1. Field of the Invention
The invention relates to a tube target for cathode sputtering installations, to a process for producing a cylindrical hollow body for such a tube target and to its use.
2. Discussion of the Prior Art
Rotating tube targets for cathode sputtering installations are known. They are primarily used for the production of coatings over large surface areas, for example for the coating of flat glass. In comparison with planar targets, tube targets permit a higher yield of target material, which is of major economic significance in particular in the case of high-value coating materials such as noble metals.
Various processes are known for the production of a target tube, or a cylindrical hollow body, from coating material. To achieve high sputtering performances with the tube target and a high degree of uniformity, or a uniform thickness of the coating produced, a uniform, fine-grained structure of the target material is aimed for.
For instance, a target tube can be produced from metal by casting a solid bar, forging the bar into a cylindrical billet and subsequently producing a tube by machining. The material yield in the case of this process is low, however, and the machining is complex. Moreover, high degrees of deformation are required to achieve a uniform, fine-grained structure.
The production of a metallic target tube may also be performed, however, by continuous casting with subsequent machining of the inside and outside diameters. Due to the slow solidification of the tube in continuous casting, however, a coarse-grained and consequently less suitable structure is produced. In the case of horizontal continuous casting, moreover, a nonuniform grain structure is formed over the tube cross section, leading to different rates of deposition.
A metallic target tube may also be produced by carrying out a forming operation on a thick strip to form a tubular geometry and closing the longitudinal seam by welding. However, the welded seam has a nonuniform structure, which differs from the material of the strip which has undergone the forming.
Production by means of extrusion of a hollow-cylindrical bolt is possible both for metallic coating materials and for ceramic coating materials. The material yield is low, however.
U.S. Pat. No. 5,354,446 describes a rotatable tube target on the magnetron principle with ceramic coating material for cathode sputtering. The target has a target holder, which has two metallic layers and, as the uppermost layer, a ceramic layer. The coefficients of thermal expansion of the two metallic layers are chosen such that a more or less smooth transition is produced between the coefficient of thermal expansion of the target holder and that of the ceramic layer. The production of the layers preferably takes place by complex processes such as plasma spraying or pressing layer particles onto the target holder by means of isostatic pressing or hot isostatic pressing.
French reference 2 745 010 describes a tube target for cathode sputtering which is composed of a plurality of elongate plates. The outer tube shape in this case preferably takes the form of a polygon. Elongate plates of standard materials in the form of metal sheets or bars may be used. The production of elongate plates from special materials can be performed by pressing, in particular by isostatic or hot isostatic pressing. The assembly of the plates to form a tube is performed by screwing, soldering or adhesive bonding.